The Secretin family consists of a group of Gram-negative bacterial outer membrane proteins that form multimeric pores through which macromolecules, usually proteins, but also filamentous phage can be secreted (Bitter et al., 1998; Cornelis et al., 1998; Hu et al., 1998; Korotkov et al. 2011). These proteins form homomultimeric ring structures, with large central pores (inner diameters of ~5 nm). The pores are plugged, and consequently conductance through secretin pores is minimal. Two secretins, PilQ of Neisseria meningitidis, and PulD of Klebsiella oxytoca are dodecamers with 12 or 14 identical subunits arranged in a ring (Collins et al., 2001, 2003; Linderoth et al., 1997). Secretin phylogeny has been studied by Nguyen et al. (2000) and Clock et al. (2008). At least some secretins are constitutively in a partially open state (Disconzi et al. 2014).

Secretins are large proteins (420-750 amino acyl residues) consisting of two domains: an N-terminal periplasmic domain (the first 280 residues of XcpQ) and a C-terminal 'homology' domain that is embedded in the outer membrane (residues 283-568 in XcpQ). The C-terminal 'homology' domains of secretins are exclusively responsible for channel formation (Brok et al., 1999) but also includes the central disc and the plug (Chami et al., 2005). The C-domain penetrates both the peptidoglycan on the periplasmic side and the lipopolysaccharide and capsule layers on the cell surface (Chami et al., 2005). A C-terminal S-domain interacts with pilotin, a protein that facilitates secretin targeting to the outer membrane. Secretin subunits, containing multiple domains, interact with numerous other proteins, including secretion-system partner proteins and exoproteins. Features common to all secretins include a cylindrical arrangement of 12-15 subunits, a large periplasmic vestibule with a wide opening at one end and a periplasmic gate at the other (Korotkov et al., 2011).

The PilQ DNA competence secretin complex (3.A.11.1.3) is 15 nm wide and 34 nm long and consists of a stable 'cone' and 'cup' five ring structure with a large central channel (Burkhardt et al., 2011). The individual rings are formed by conserved domains of alternating α-helices and β-sheets. The PilQ complex spans the entire cell periphery of T. thermophilus, consistent with the hypothesis that PilQ accommodates a PilA4 comprising pseudopilus, mediating DNA transport across the outer membrane and periplasmic space in a single-step process (Burkhardt et al., 2011).

Martínez, A., P. Ostrovsky, and D.N. Nunn. (1998). Identification of an additional member of the secretin superfamily of proteins in Pseudomonas aeruginosa that is able to function in type II protein secretion. Mol. Microbiol. 28: 1235-1246. 9680212